Halt build on YAML error without jinja2

This fixes the issue described in both #247 and #137 by detecting parse errors related to "{{" inside a YAML file.  The major reason for those to be there is when doing template work.
According to [the docs][1], all user-land exceptions should extend from `Exception`.  I'm not familiar with the best practice on extending from other exceptions.

[1]: https://docs.python.org/2/library/exceptions.html#exceptions.Exception